E ﬀ ects of Cultivating Rice and Wheat with and without Organic Fertilizer Application on Greenhouse Gas Emissions and Soil Quality in Khost, Afghanistan

: The agricultural sector is the most important economic component in Afghanistan, as 80% of the population is involved. The improvement of cereal production is an urgent task to meet the nation’s demand for the staple within the limited arable land. To promote a sustainable crop production system, this study examined the soil quality to learn the basic knowledge of soil fertility and the environmental impact of di ﬀ erent rice–wheat cropping systems in Khost, Afghanistan by using the life cycle assessment (LCA) method. The economic analysis of each farming system was conducted by the data gathered by the farmers’ interviews along with LCA data collection. The analysis considered the on-farm activities, which were required to produce 1 kg of wheat and rice. It included energy use, production, and farming inputs such as fertilizer and agrochemicals. Conventional farming with organic fertilizer application (CF + OF) was compared with conventional farming (CF). The LCA results showed the total greenhouse gas (GHG) emission was higher in rice production compared to wheat production. However, CO 2 absorption by the crops was far greater than the total GHG emission in both systems and showed great potential for soil carbon sequestration for mitigation of global warming. The soil examination revealed the CF + OF system increased soil total carbon (TC), active C (AC), total N (TN), soil organic carbon storage (SCS), P, and K + after four years of organic fertilizer application. The yield of each crop was slightly higher in the CF system; however, the CF + OF system increased net income by reducing the cost for fertilizer. The study concluded the CF + OF system can improve soil fertility in the long term while saving the farming operation cost. Further research is required to determine the best combination of practices to improve cattle manure characteristics and farm management for soil carbon sequestration to promote a sustainable farming system in the country.


Introduction
Afghanistan is a 65 million hectare land-locked nation, mostly occupied by high mountains (80%) and desert. Decades of war destroyed infrastructure and decimated economic development including the agricultural sector. The agricultural sector is the most important component of the country's economic development, as 80% of the population is engaged in agriculture [1]. Arable land is quite limited at only 7.5 million hectares (12%), of which 3.2 million ha are irrigated and 4.8 million ha are rain-fed [2,3]. Due to topographical conditions and lack of water, grain production has been intensively practiced in the north mountain range or around the Kabul river basin in the southeast part acknowledged standardization and processing. Moreover, the cost of inorganic fertilizers has been increasing enormously [28], to the extent that they are out of reach for small and marginal farmers. The current conventional agriculture in Afghanistan is under threat. However, the potential domestic production could alter this scenario toward self-sufficiency through appropriate farming practices, fertilizer management, agricultural policies, and technical knowledge of production management. Therefore, the hypothesis created in this study is that the regular organic manure application, along with chemical fertilizers, could improve soil quality and crop production in the region where there are limitations in agricultural and technical resources.
Khost province is one of the agricultural areas that can produce two crops per year thanks to irrigation systems [31]. Our former study showed the retention of plant residue improved soil water retention and bulk density [32]. However, there is little study on the sustainability of small-scale farming in this region. According to the National Renewable Energy Laboratory report, this region produces more than 75,000 tons of crop residues per year [9] and could have the potential to increase crop yields by using organic fertilizers.
In addition, lack of soil property data is one of the major constraints for local farmers as well as other groups involved in agricultural development in Afghanistan. There was also an absence of available data analyzing the impacts on GHG emissions of organic fertilizer application in Afghanistan. To promote sustainable agriculture in Afghanistan and improve soil quality and cropping systems, it is important to gather the basic knowledge of the regional soil properties, the environmental impacts of current farming practices, and the costs of a new farming method for adaptation to climate change.
Therefore, the objectives of this study are: (1) to evaluate the environmental impact on GHG emissions by life cycle assessment (LCA) of the current farming systems; (2) to present the information of soil quality in the region; and (3) to analyze the economic status of rice and wheat double cropping systems toward sustainable agricultural production.

Description of the Rice-Wheat Double Cropping System Analyzed in This Study
Khost province is one of the major wheat and rice producers in the country, covering an area of 4029 km 2 . Figure 1 illustrates the geographical location of the study sites. It is located~1180 m above mean sea level between 33 • 59 -33 • 46 North latitudes and 69 • 19 -70 • 21 East longitudes in southeast Afghanistan. Around two-fifths (59%) of Khost province is mountainous or semi-mountainous terrain, while more than one-third (37%) of the area is flat. Khost boasts 123,500 ha of forest and has 14,911 ha of agricultural land [33]. Farmers in this region grow various crops including wheat, rice, corn, and different vegetables. The main crops are wheat (Triticum aestivum L.), maize (Zea mays L.), rice (Oryza sativa L.), and clover (Trifolium spp.). Wheat is cultivated in the autumn (mid-October), whereas maize and rice are grown in the summer (mid-April-mid-May) [4].
This region is a typical semi-arid climate with 478 mm annual rainfall [31]. The mean annual rainfall and temperature in 2016 are shown in Figure 2. The study area has dynamic climatic and agricultural settings compared to other provinces of Afghanistan. For example, the high temperature (39-42 • C) remains in the summer due to warm breezes from the north, while the temperature drops up to −4 • C in the winter due to cool breezes coming from the east [33].

Data Collection and Farming Practices
The field investigations and interviews with farmers were conducted to obtain farming practice details in June 2016. Data were collected through face-to-face interviews with 4 farmers in the local language (Pashto) on farm size, cultivars of crops, material inputs, cultivation methods, agronomic practices, harvest, processing techniques, weed control, fertilizer application methods, yield, gross income, and total expenditure in rice and wheat production. The collected data were used to conduct Life Cycle Assessment (LCA) and economic analysis of these two farming systems.
The farmers in this region apply 150 kg ha -1 di-ammonium-phosphate (DAP) and 200 kg ha -1 urea for wheat and rice production under the conventional farming (CF) practice. The other farmers use cattle manure (7.5 t ha -1 ) along with a half dose of the chemical fertilizers under the conventional farming with organic fertilizer application (CF+OF) practice ( Figure 3). The CF+OF farmers have been using cattle manure for four years. The nutrients of this farm-made cattle manure were 31.8% total C, 1.2% total N, 0.75% K2O, 0.18% P2O5, 0.11% MgO, and 0.13% CaO, with a C:N ratio of 25.9.
In Khost province, farmers perform conventional rotary tillage to a depth of up to 15 cm prior to sowing any crop. In small fields, farmers manually plow by shawl. Most farmers use disk harrows for breaking clods and removing plant residues from the fields. During the growing season, farmers use hoes (locally called rambai) and hand weeding to remove weeds from vegetable and paddy fields, respectively. Generally, farmers in this area grow wheat using broadcasting methods and flood irrigation systems. In this area, the planting period for wheat cultivation starts in November and the harvest is in June. Paddy rice is grown by hand transplanting rice seedlings that are provided from their mature plants, neighbors, the local market, or agricultural organizations. In this area, the planting period for rice cultivation is from May-June, and harvest occurs at the end of October. Rice plants take approximately 130-160 days to mature from planting to harvesting, depending on the cultivars and field locations (Table 1).

Data Collection and Farming Practices
The field investigations and interviews with farmers were conducted to obtain farming practice details in June 2016. Data were collected through face-to-face interviews with 4 farmers in the local language (Pashto) on farm size, cultivars of crops, material inputs, cultivation methods, agronomic practices, harvest, processing techniques, weed control, fertilizer application methods, yield, gross income, and total expenditure in rice and wheat production. The collected data were used to conduct Life Cycle Assessment (LCA) and economic analysis of these two farming systems.
The farmers in this region apply 150 kg ha −1 di-ammonium-phosphate (DAP) and 200 kg ha −1 urea for wheat and rice production under the conventional farming (CF) practice. The other farmers use cattle manure (7.5 t ha −1 ) along with a half dose of the chemical fertilizers under the conventional farming with organic fertilizer application (CF+OF) practice ( Figure 3). The CF+OF farmers have been using cattle manure for four years. The nutrients of this farm-made cattle manure were 31.8% total C, 1.2% total N, 0.75% K 2 O, 0.18% P 2 O 5 , 0.11% MgO, and 0.13% CaO, with a C:N ratio of 25.9.   In Khost province, farmers perform conventional rotary tillage to a depth of up to 15 cm prior to sowing any crop. In small fields, farmers manually plow by shawl. Most farmers use disk harrows for breaking clods and removing plant residues from the fields. During the growing season, farmers use hoes (locally called rambai) and hand weeding to remove weeds from vegetable and paddy fields, respectively. Generally, farmers in this area grow wheat using broadcasting methods and flood irrigation systems. In this area, the planting period for wheat cultivation starts in November and the harvest is in June. Paddy rice is grown by hand transplanting rice seedlings that are provided from their mature plants, neighbors, the local market, or agricultural organizations. In this area, the planting period for rice cultivation is from May-June, and harvest occurs at the end of October. Rice plants take approximately 130-160 days to mature from planting to harvesting, depending on the cultivars and field locations ( Table 1).

. Goal and Scope Definition
LCA is one of the environmental assessment tools used to quantify and evaluate the environmental impacts of a product through all stages in its life cycle. According to ISO 14040 [34], an LCA comprises four steps: goal and scope definition, life cycle inventory, life cycle impact assessment, and interpretation of the results. The goal of this study is to generate a quantitative environmental profile of the cropping system in the Khost province of Afghanistan. This study aims to (1) present a full chain analysis of farm activities in the rice and wheat cultivation, and (2) compare it with the conventional farming system in the aspect of the total GHG emissions to analyze the impact on global warming. The sum of CO 2 , CH 4 , and N 2 O emissions from each farming system were calculated from the total use of energy (fuels and electricity) and the amount of fertilizers, agrochemicals, and plastic wastes [35]. The global warming impact is evaluated in terms of global warming potential (GWP) over a 100-year time horizon and presented in units of kg CO 2 equivalent. The CH 4 and N 2 O emissions from the cultivation system were converted into CO 2 emissions by using the global warming potential of each gas (CH 4 : 21, N 2 O: 310) [36]. Carbon dioxide absorption by crops was calculated by using the following equation [35]. (1) The cultivation system investigated in this study is a double cropping system to produce rice and wheat with organic fertilizer as an alternative cropping system. The results of the LCA, when combined with economic and soil analysis, will provide sufficiently broad information to be used by the agricultural extension sector of the Khost provincial government to understand the issues of the current farming system, namely to provide suggestions of better yields and soil quality for the future promotion of a sustainable farming system.
As the main function of the system under this study is to produce rice and wheat as the main staples, the functional unit selected is 1 kg of each grain. The scope of assessment is limited to the arable land (cradle-to-farm gate), excluding the manufacturing, packing, and transportation of raw materials (agrochemicals and farming tools such as nursery boxes and grain bags). The system boundaries of each cropping system are shown in Figure 4. This study covers the entire life cycle of farm activities including the field preparation, seeding, field management, harvesting, and grain adjustment before shipping to the market. For the LCA analysis, the environmental impacts considered include abiotic resource depletion (the fuel consumption), global warming (nitrogen input and agrochemical use), and land use (crop yields). These data were obtained from the interviews described in Section 2.2. The costs of each crop production were also calculated to optimize eco-efficiency as a financial function. The details of GHG emission basic units used for LCA calculation is described in Supplementary Materials. Table 2 shows the main process of the life cycle of rice and wheat production and their sources of data. Due to the limitations of data collection, agricultural materials such as nursery boxes and manual tools that could be used for more than one year were excluded from the LCA calculation. Plastic materials disposed of within one year were counted as plastic waste.   The life cycle inventory (LCI) is basically the collection of the data. This involves data collection for inputs and outputs of the crop production system gathered by the interviews. Some data were

Inventory Analysis
The life cycle inventory (LCI) is basically the collection of the data. This involves data collection for inputs and outputs of the crop production system gathered by the interviews. Some data were taken from literature sources. The farmers work only in the daytime due to the limited access to electricity in the region. Therefore, there was no use of electricity in the cultivation system. All the input and output data used in the production systems is listed in Table 3. Table 3. Life cycle inventory (LCI) material input and output data of the two cropping systems. The figures in Energy consumption, Production cost and Nitrogen inputs are the average of total rice and wheat production used in 2016. The data are presented as the average of two farms in each system. The input and output data were collected by the interviews in Section 2.2. The costs of fuel and fertilizers were converted into US dollars from Afghani (1 AFN = 0.013 USD).

Soil Sampling
To understand the effects of organic fertilizer application, four replications of soil samples were collected from each field by sinking a 5-cm diameter steel cylinder into the soil to a depth of 30 cm. Each soil core sample was then sliced with a sharp knife into two layers of 0-15 and 15-30 cm depths. The soil samples were placed in paper bags and dried at room temperature for one week. The dry samples were passed through a 2 mm sieve to prepare them for chemical analyses (total carbon, active carbon, inorganic N, K + , Mg 2+ , Ca 2+ , and P). To measure carbon content, subsamples were further dried at 105 • C for 72 h and measured using a CN analyzer (JM3000N/CN, J Science Lab Co. Ltd., Kyoto, Japan). The soil active carbon (AC) concentration was measured from 2.5 g air-dried samples combined with 20 mL of 0.02 mol KMnO 4 solution and shaken for 2 min. The absorbance of each sample was then recorded using mass spectrometry at 550 nm [38]. AC was determined as: where Ci is the initial solution concentration (0.02 mol L −1 ), a is the intercept, b is the slope of the standard curve, MC is the mass of carbon (9000 mg, 0.75 mol) that is oxidized from Mn 7+ to Mn 4+ by 1 mol of MnO 4 , Vsol is the volume of KMnO 4 solution (0.021 L), and Ws is the soil weight (0.0025 kg).
To measure soil inorganic nitrogen, 5 g soil samples were combined with 40 ml of 1 mol L −1 KCl and shaken for 1 h. Absorption was measured using UV-visible absorption spectrophotometry [39] and the indophenol blue method [40] for soil NO 3 -N and NH 4 -N, respectively. To measure soil exchangeable K + , Mg 2+ , and Ca 2+ , 2 g air-dried soil samples were combined with 40 mL of 1 mol L -1 ammonium acetate and shaken for 1 h. To measure soil available phosphorus, 0.2 g air-dried soil samples were combined with 40 mL of 0.002 N sulfuric acid and shaken for 30 min. The nutrient concentrations were then measured using a soil and plant clinical analyzer (SPCA-6210, Shimadzu, Kyoto, Japan). Soil pH was measured using a LaMotte field kit. Soil texture was determined using the Gee and Bauder pipette method [41], and soil organic carbon storage (SCS) was calculated using the following equation.

Statistical Analysis
The statistical analyses were performed using STAT View (STATView for Windows, version 5; SAS Institute, Cary, NC) [42]. By using the soil property data of the four samples in the four farms, Analysis of variance (ANOVA) was performed within each treatment, and means were compared using Tukey-Kramer test.

Economic Analysis
To understand the cost benefits of each farming system, benefit-cost ratio was used as an indicator for the cost-benefit analysis to summarize the overall value for money of the farming practice [43]. The data collected by the farmer's interviews, such as the yield of crop, the sales price, gross/net income, and the total expenditure, were used for the calculation. The data was obtained as Afghani (AFN), the local currency, and converted into United States Dollar (USD) with the exchange rate on 1 June 2020 as 1 AFN = 0.013 USD.

Life Cycle Impact Analysis
The GHG emissions per kg of grain in the CF+OF system were 0.53 kg-CO 2 for wheat and 0.87 kg-CO 2 for the rice production. In the CF system, the GHG emissions were 0.32 kg-CO 2 for wheat and 0.96 kg-CO 2 for rice. The total CO 2 emissions were higher in the rice cultivation than those in the wheat production. The fuel consumption was the major contributor of GHG emission in wheat production. On the other hand, in the rice production systems, CO 2 emissions from the soil accounted for almost half of the total CO 2 emissions because of CH 4 emission from paddy fields ( Figure 5). The statistical analyses were performed using STAT View (STATView for Windows, version 5; SAS Institute, Cary, NC) [42]. By using the soil property data of the four samples in the four farms, Analysis of variance (ANOVA) was performed within each treatment, and means were compared using Tukey-Kramer test.

Economic Analysis
To understand the cost benefits of each farming system, benefit-cost ratio was used as an indicator for the cost-benefit analysis to summarize the overall value for money of the farming practice [43]. The data collected by the farmer's interviews, such as the yield of crop, the sales price, gross/net income, and the total expenditure, were used for the calculation. The data was obtained as Afghani (AFN), the local currency, and converted into United States Dollar (USD) with the exchange rate on 1st June 2020 as 1 AFN = 0.013 USD.

Life Cycle Impact Analysis
The GHG emissions per kg of grain in the CF+OF system were 0.53 kg-CO2 for wheat and 0.87 kg-CO2 for the rice production. In the CF system, the GHG emissions were 0.32 kg-CO2 for wheat and 0.96 kg-CO2 for rice. The total CO2 emissions were higher in the rice cultivation than those in the wheat production. The fuel consumption was the major contributor of GHG emission in wheat production. On the other hand, in the rice production systems, CO2 emissions from the soil accounted for almost half of the total CO2 emissions because of CH4 emission from paddy fields ( Figure 5).
The emission from the LCI of each cropping system was presented in Table 4. CO2 absorption by crop was greater than the total GHG emission. In total production of the two crops, 11.14 Mg ha -1 of CO2 was absorbed by the CF system, and 10.48 Mg ha -1 of CO2 was absorbed in the CF+OF system. The GHG balance of the two crops in the CF was 637 kg CO2 greater in the CF because of higher yields. The emission from the LCI of each cropping system was presented in Table 4. CO 2 absorption by crop was greater than the total GHG emission. In total production of the two crops, 11.14 Mg ha −1 of CO 2 was absorbed by the CF system, and 10.48 Mg ha −1 of CO 2 was absorbed in the CF+OF system. The GHG balance of the two crops in the CF was 637 kg CO 2 greater in the CF because of higher yields.
Total GHG emission shows the sum of CO 2 emissions from fuel, plastic wastes, and fertilizers and agrochemicals per crop. The W+R (wheat and rice) sum counted the figures of GHG emission and CO 2 absorption in the entire double cropping system. CO 2 absorption of each crop was calculated by the Equation (1). Negative quantity means CO 2 absorption was greater than GHG emissions.

Total Carbon, Active Carbon, and Total Nitrogen
The type of farming system significantly affected TC, AC, and total N (TN). TC and TN were also significantly different between depth layers, but AC was not affected by soil depth. In the 0-15 cm layer, TC, AC, and TN were significantly higher in the CF+OF system than the CF system (Table 5); TC concentration was 29.5% higher in CF+OF than CF. A similar trend was observed for AC and TN in the 0-15 cm layer, in which they were 18% and 45.5% higher in CF+OF than CF, respectively. In the subsoil layer (15-30 cm), TC, AC, and TN concentrations were 31.8%, 6.1%, and 44.4% higher in CF+OF than CF, respectively. The soil C:N ratio was higher in CF than in CF+OF in the in 0-15 cm layer, but in the 15-30 cm soil layer, the C:N ratio did not differ between farm types.

Soil Chemical Properties
Soil inorganic N content showed significant differences between farming systems but did not significantly vary with soil depth; the interaction effect was also significant ( Table 5). In the 0-15 cm layer, the inorganic N content was 55 mg kg −1 in CF, which was 7.2% higher than in CF+OF. The highest inorganic N content was recorded in the 15-30 cm layer in CF, which was 33.2% higher than in CF+OF. The farming system (FS) significantly affected soil P, and the interaction of FS × depth was also significant. The CF+OF system produced significantly higher soil P in both layers. In the 0-15 and 15-30 cm layers, P concentration was 77.9% and 109.9% higher in CF+OF than CF, respectively. The soil potassium (K + ) content showed no significant variation between FSs and soil depths. The farming system significantly affected Ca 2+ and Mg 2+ contents, but the depth layer and the FS × depth did not impact on these contents. In both layers, the soil Ca 2+ content was higher in CF+OF than in CF (6.3%, 0-15 cm layer; 7.1%, 15-30 cm layer). There was the same trend in the soil Mg 2+ content (4.2%, 0-15 cm layer; 3.0%, 15-30 cm layer). The farming system did not affect soil pH and showed the character of alkaline soil.
The soil textural class was sandy loam, and there was no significant difference between farming systems ( Table 6). The CF+OF soil had higher silt content than CF, while the CF sand and clay contents were higher than in CF+OF. There were no significant differences in soil bulk density between the two farming systems. The soil carbon content was higher in the CF+OF system than in the CF system. The SCS in the CF+OF system increased by an average of 27.5% compared to the CF system. Table 5. Comparison of soil pH, total carbon (TC), active carbon (AC), total nitrogen (TN), C:N ratio, inorganic N, P, K + , Ca 2+ , and Mg 2+ in conventional farming (CF) and conventional farming with organic fertilizer (CF+OF) systems in Khost Province, Afghanistan. The figures of AC were obtained by Equation (2)  The correlation coefficients of the soil properties are presented in Table 7. TC was significantly positively correlated with AC, TN, P, and K + and negatively correlated with C/N, inorganic N, and Mg 2+ . AC showed a significant positive correlation with TN and P and a negative correlation with the other soil properties. TN also exhibited a significant positive correlation with P and K + . C/N was significantly negatively correlated with K + and Ca +2 . P was significantly positively correlated with Mg +2 . Note: *, **, and *** denote significant differences at p < 0.05, p < 0.01, and p < 0.001, respectively.

Wheat and Rice Yield and Economic Analysis
The economic analyses of wheat and rice production for both FSs are shown in Table 8. The rice yields were slightly greater in the CF system as 3.7 t ha −1 , while the wheat yields were not affected by the FS. The CF+OF farming system reduced the cost of wheat and rice production. The CF farmers paid an average of 253.5 USD for chemical fertilizers, while the CF+OF farmers paid an average of 187.5 and 182.2 USD for wheat and rice production, respectively ( Figure 6). Therefore, the CF+OF system can reduce fertilizer costs by 26.0% and 28.1% for wheat and rice production, respectively. The average total expenditure for farmers with CF were 572.7 and 822.3 USD for wheat and rice production, respectively, compared to an average of 536.6 and 762.6 USD for wheat and rice production in the CF+OF system, respectively (Table 8). Therefore, the farmers with CF+OF systems could reduce total costs by 6.3% and 7.2% for wheat and rice production, respectively. For wheat production, the average net income was 2.4% higher in CF+OF than CF, while this difference was not observed for rice production. CF+OF systems required more labor to apply cattle manure to the fields.
Sustainability 2020, 12, x FOR PEER REVIEW 2 of 21 Table 8. Comparison of wheat and rice production economic analyses between the conventional farming (CF) and the conventional farming with organic fertilizer (CF+OF) systems in Khost Province, Afghanistan.

Interpretation of LCA
The GHG emissions per kg of rice were higher than those in wheat because paddy fields produce CH 4 during the cultivation period. The GHG emissions from fertilizers and agrochemicals are considered low in general compared to India or the United States, where the use of chemical fertilizers is more than tenfold compared to Afghanistan [44]. CO 2 absorption by crops was greater than the total GHG emission in both cropping systems. Livestock manure provides a good source of essential nutrients for plants and is an excellent agent for improving soil quality and productivity. Although manure is the second largest source of GHG emissions in the agricultural sector [45], this amount of cattle manure application in the study had a benefit to soil improvement rather than an environmental impact. Shahzad et al. [46] reported that the capacity of carbon sequestration by cattle manure application was 1-1.2 Mg h −1 higher than chemical fertilizer application [46]. A long-term application of manure can increase the capacity of soils to act as a carbon sink when soil quality is low [47]. In addition, since N 2 O emissions in crop fields directly correlate to chemical fertilizer application [48], the use of less chemical fertilizer naturally contributes to reduce N 2 O emissions as well as farming operation costs. This data revealed that GHG emissions per wheat yield were greater in the CF+OF system; however, for rice production, the CF system produced more GHG emissions. The CF+OF crops absorbed less CO 2 than the CF crops ( Figure 5). Aguirre-Villegaset et al. reported that efficient manure management is essential for GHG emission reduction [47]. Many studies show evidence that mid-season drainage significantly reduces CH 4 emissions and exerts a positive impact on rice yields by increasing N mineralization in the soil and increasing rice plant root development [49][50][51]. In the CF+OF system, additional use of tractors was required to bring cattle manure into the field and for mixing manure into the soil, resulting in more fuel consumption that directly affected the GHG emissions. In these farming systems, another approach is required to reduce the amount of CO 2 emission per hectare. In terms of energy consumption, mechanical systems are more energy efficient than diesel-based tractor systems for manure operations [47]. Future research is needed to assess the effects of manure management strategies on GHG emissions in this region.

Changes in Soil Properties by Organic Fertilizer Application
Greater organic matter input increases SOC and AC in the surface layer [32]. In this study, TC and AC in the 0-15 cm soil layers were significantly higher in the CF+OF system than CF (Table 5). In the CF+OF system, farmers used organic fertilizer (cattle manure) for four years and, as a result, TC and AC contents were significantly increased, suggesting that cattle manure application contributed to improved soil carbon storage. This increase was associated with a greater amount of organic fertilizer input; thus, the organic fertilizer amendment was adequate for improving soil quality in Afghanistan's soil. Based on the data obtained from the farmers' interviews, the farmers used pure cow dung as organic fertilizer in their field. To increase future soil organic carbon content in soil, they should mix plant residues with their cow dung to make it rich in SOC contents and avoid crop failure. The distribution of total nitrogen content followed a similar trend to the TC distribution and was relatively higher in the CF+OF system than TC in the CF system (Table 5). This result is consistent with Zibilske et al. [52], who reported that changes in soil organic N are often directly related to changes in soil organic C. Overall, the SOC level in Afghanistan is lower than in other Asian countries [12,53], which may be attributed to the low organic input and low rainfall in the region. In addition, the wet and dry cycles caused by intermittent irrigation might enhance soil organic matter decomposition, resulting in low TC, AC, and TN levels. Due to the lack of energy source in Afghanistan, farmers use animals dungs as a fuel source; therefore, the animal manure usage for crop production is not sufficient to produce a high yield [54]. According to the interviews, rice and wheat straw are removed from the fields and fed to animals in this region. From this brief observation, this study suggests that farmers have to grow separate crops for animal feeding on a crop rotation base and return plant residue to soil, which will contribute to improving soil quality and protecting soils from erosion and degradation.
The change in the soil carbon storage ability among farming systems was also observed ( Table 6). The CF+OF system increased the SCS per hectare compared to the CF system. This result is consistent with Komatsuzaki and Syuaib's study [55], which reported that organic farming can increase SCS by 1.85-Mg per hectare per year compared with conventional farming systems in paddy fields in Indonesia. These results demonstrate that cattle manure application has the potential to improve soil carbon sequestration in Afghanistan. In general, if organic manure or crop residues are returned and supplemented with nutrient inputs, these systems can maintain adequate soil organic matter and production levels [56]. However, in the CF system, chemical fertilizer input only supplies nutrients to crops and does not improve soil quality [57]. Thus, this study revealed that the SOC contents in the CF+OF farming system were higher than in the CF system, due to the input of appropriate organic matter such as cattle manure.
Soil carbon, nitrogen, potassium, and phosphorus are among the most important plant essential elements because the soil C:N:K:P ratio can reflect soil fertility and plant nutritional status; changes in the content and ratio of these elements can affect the growth and distribution of vegetation [58]. The ratio of soil C, N, and P is strongly influenced by soil type, vegetation community characteristics, climatic conditions, and the vegetation development stage [59,60]. In this study, TC and TN had a significantly positive correlation between soil P and K + ( Table 7). C:N ratio is an index of nutrient mineralization and immobilization, where a low C:N ratio indicates higher rates of mineralization [61]. In the study, the C:N ratio was found to be higher than the normal range of 10:1 that is expected in mineral soils [62]. However, Chesworth [63] reported that nitrogen is immobilized at higher C:N ratios due to the formation of only slightly biodegradable complexes that are low in nitrogen. Thus, in this study, C:N ratio had a negative correlation with soil P and K + .
Soil pH did not significantly differ across farming systems or soil depths. The soil pH in the 0-15 cm soil layer (8.5) was higher than in the 15-30 cm soil layer (Table 5). These results suggested that changes in pH were not detected in the short term [64]. Soil inorganic N content was significantly different between farming systems. In both systems, higher inorganic N was observed in the subsoil layer. The higher observed inorganic N in the subsoil layer might be due to nitrate leaching from the surface soil. In both layers, the CF+OF farming soil contained higher soil P than the CF soil, which is likely related to the application of cattle manure in these fields. Moreover, farmers have been applying DAP fertilizer to the fields. The lower P content might be related to phosphorus fixation [64], although the averages were lower than those reported in tropical Asia [12]. The low P availability was probably due to the scarcity of P from fertilizer or other amendments to the soil and P fixation by calcium under alkaline soil conditions. David reported that pig manure application increased soil K + concentration [65]. In this study, the 15-30 cm layer in the CF+OF system had slightly higher K + than in the CF system. The observed highest concentration of K + was attributed to the application of household wastes like cattle manure. The observed highest concentration of Ca 2+ in this area was attributed to the high calcium carbonate content and high pH caused by carbonate-rich geology and dry climate conditions. The CF+OF system's 15-30 cm layer also had higher Mg 2+ than the CF system.

Challenges to Overcome Low Crop Productivity
In general, crop residues along with nutrient inputs maintain adequate soil organic matter and plant production levels [66]. However, removing crop residue for animal feeding is a common practice in conventional farming systems in Khost province. In this system, only the crop root systems are recycled and as a result, soil fertility becomes very low. This is why farmers use chemical fertilizers and perform tilling for weed control to achieve optimum yield. According to this data, the average rice yield (3.6 t ha −1 ) was lower than in another Asian country where 200:50:50 N-P 2 O 5 -K 2 O kg ha −1 fertilizer obtained 4.3 t ha −1 rice yield [66]. In Pakistan, a study reported that using 120 kg ha −1 N, 90 kg ha −1 P and 60 kg ha −1 potash increased wheat yield (4.6 t ha −1 ) [67]. In this study, the average wheat yield in Khost province was 3.8 t ha −1 . This result suggests that due to lack of soil fertility, the application of essential plant nutrients is required to achieve adequate yield. In this region, beside soil degradation, some other problems seem to directly affect crop production. Kakar et al. [28] reported that the majority of farmers saved self-produced seed for future rice cultivation and suggested that government should lead the way in the distribution of improved rice cultivar to farmers throughout the country. Lal [18] also proposed that improved varieties with fertilizer input under better irrigated conditions can increase wheat and rice crop yield in Afghanistan. To maintain long term soil fertility and crop productivity, adding organic fertilizer to the fields is a very practical way to increase soil organic carbon in Afghanistan. In addition, making organic fertilizer from livestock has several benefits, such as reducing organic wastes and the cost of fertilizer.

Economic Advantages of Organic Fertilizer Application
The cost of the CF system is on average twice as expensive as is it for organic products, while production levels are almost the same [68]. The wheat and rice yields were higher in the CF than the yields in the CF+OF system, but the cost of chemical fertilizers reduces the net income of the CF farmers. For example, the net income for wheat crops was 2.4% higher for the CF+OF farmers than the CF farmers (Table 8). This indicates that the CF+OF system reduced the chemical fertilizer cost, increasing the farmers' income in Afghanistan. In the past comparison study of farming systems in Indonesia, organic farming was able to cut 90% of the total cost of rice production in West Java region [55]. In Khost province, the cost of organic fertilizers (cattle manure) is very reasonable compared to that of chemical fertilizers because the local farmers keep a couple of cows in their farms and their dung can be utilized as fertilizer. A double amount of organic fertilizer could have the potential to increase more soil carbon and change soil properties in the long term. However, crop residues as a secondary material for cattle manure would compete with animal feed in this region. For further improvement of soil quality, another approach, such as the use of weeds or pasture, is required along with cattle manure application for better manure quality. The quality of cattle manure is very critical for soil management because the timing of the manure application is limited in the double cropping system. The appropriate manure management technology needs to be transferred to the local farmers. Since the low soil productivity in the Afghan farmland has been a serious issue for years, this study was able to present an example to improve soil quality using a practical and economical approach for small-scale farmers. This research is limited to the discussion of GHG emissions, soil quality, and cattle manure management systems in Afghanistan. Therefore, further research is required to determine the best combination of farming practices to improve soil quality and crop yield in the long term. In addition, a better way to increase good quality of organic fertilizer is needed for the farmers to shift to better economical farming practices.

Conclusions
A comparative LCA of double cropping systems, including conventional farming and conventional farming with organic fertilizer, was conducted along with soil and economic analysis to provide sufficiently broad information to understand the issues of the current farming system in Khost province in Afghanistan. Since there is little study on environmental impacts of the current farming system, this study aimed to provide practical suggestions for better yields and soil quality for the future promotion of a sustainable farming system. The LCA results revealed that conventional farming with organic fertilizer reduced the GHG emissions per kg of grain in rice production. CO 2 absorption by rice and wheat grain production was greater than total GHG emission in both cropping systems. The soil analysis discovered that four years' use of cattle manure improved soil total C, active C, total N, P, Ca 2+ , and Mg 2+ content. In addition, cattle manure may help not only to improve soil quality by increasing carbon sequestration, but also to increase crop production and to create sustainable food systems in Afghanistan. In the economic aspect, conventional farming with organic fertilizer system was able to cut the average cost of farming by 6.7% while producing almost the same crop yields compared with the conventional farming system. However, further research is required to determine the best combination of practices (ratio of organic fertilizer and chemical fertilizer), manure characteristics, and management systems to reduce GHG emissions and farming operation costs. Social challenges such as the adaptation of new technologies, improvement of irrigation systems, and application of organic fertilizers may obstruct sustainable farming in Afghanistan. However, the best agronomic approach and cooperative systems based on scientific research may help to increase rice and wheat production in this region. Therefore, the collaboration with researchers, the extension sector, and the local farmers will become increasingly important to establish a sustainable crop production system in Afghanistan and to ensure food security for the nations.
Supplementary Materials: The following are available online at http://www.mdpi.com/2071-1050/12/16/6508/s1, Table S1. GHG emission basic unit per energy, Table S2. CH 3 and N 2 O emission during the grain cultivation,  Table S3. GHG emission basic unit from plastic waste, Table S4. GHG emission basic units from different type of fertilizers.
Author Contributions: M.K. designed this study and funded the project. R.H. contributed toward the interviews, lab measurements, and writing. E.M. conducted LCA analysis and edited the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the "Priority and Strategy Project of Universities" of MEXT, JAPAN.